The call setup and cleardown between a communications system and communication terminal equipment, as well as, subscriber performance features, for example, call rerouting, are controlled in communication terminal equipment using a signalling procedure. As used herein the term "cleardown" refers to clearing a call, that is taking a call off line and releasing the central office. The signalling procedure that is usually realized in program-oriented terms is controlled by key actuations in the framework of the call setup or cleardown or during the communication phase, whereby the controlling occurs before every branching within the signalling procedure. A plurality of subscriber performance features are offered with an increasing tendency, particularly in private communication networks. A plurality of special keys are provided for the control of the subscriber performance features in the associated communication terminal equipment in addition to the numerical keyboard. Such communication terminal equipment is known from the product publication "ISDN im Buero" of Siemens AG, reprint of "Telcom Report" and from Siemens-Magazin COM, page 42. Furthermore, numerous arrangements or implementations of the special keys are known for the initialization and control of the subscriber performance features. In one of these versions, keys are multiply occupied in order to save keys and, thus, costs, i.e. the individual keys have a plurality of control possibilities allocated to them, whereby the switching for all or in groups is effected by further special keys.
Further, U.S. Pat. No. 4,659,876 discloses an "audiographic communication system", whereby the subscriber performance features are initialized by touching a touch-sensitive layer of a picture screen means, what are referred to as "touch screen" keys. In all of such communication terminal equipment, a plurality of additional, special keys that are to be actuated as a function of different operating and signalling statuses of the communication terminal equipment are required for a plurality of implemented subscriber performance features. This is particularly the case in private communications networks. This type of actuation assumes extensive knowledge, particularly in view of the signalling statuses and of the possible actuation versions. As a result of this required extensive knowledge, subscriber acceptance of such communication terminal equipment is considerably reduced and use is not made of the subscriber performance features that are offered.